Light Emitting Diodes (LEDs) are gaining great popularity as light sources in various electrical applications. In many applications, LED lighting with a high dimming ratio is required, such as for realizing a high contrast display. Short circuit protection is also required for safety and reliability. In most battery powered devices, the battery is desired to be disconnected from the power input ports of the device when the system is off to prevent power leakage, i.e. complete input disconnection is required. In the prior art, a buck or boost type DC-DC converter may usually be used as a LED driving circuit. When conventional step up converter is used, at least two additional switches are needed to realize high dimming ratio control, short circuit protection, and input disconnection separately.
FIG. 1(a) shows one prior art solution to realize high dimming ratio control, short circuit protection, and input disconnection functions when a conventional boost converter is used to drive LEDs. Two additional switches S3 and S4 are inserted to the conventional boost converter topology. One end of the switch S3 is coupled to the anode of the series LEDs, and an opposite end of the switch S3 is coupled to ground. The switch S4 is connected in series with the boost converter rectifier switch S2. One skilled in the art will understand that the rectifier switch S2 can also be replaced by a diode D1. In normal operation, the boost converter power switch S1 and rectifier switch S2 are turned on complimentarily, both the additional switches S3 and S4 are in continuous conduction. When dimming, a dimming signal is provided to control the switch S3. When the dimming signal is on, the switch S3 is turned on so that power is delivered from a power supply Vin to the LED load. When the dimming signal is off, the switch S3 is turned off and thus the current flowing through the LEDs is cut off immediately. Meanwhile, the voltage across the output capacitor Co is maintained since there exists no discharge path when the switch S3 is off. Because the capacitor Co voltage is held during the dimming off period, when the dimming signal is on again, the current through the LEDs can resume to a regulated level quickly.
Therefore, with the addition of the switch S3, the current driving the LEDs can be controlled to be a square waveform, whose average value is proportional to the duty of the dimming signal even when the dimming duty is very small. In other words, high dimming ratio can be achieved. When short circuit or over current conditions are detected at the output of the converter, for example the output current is detected to have reached a protection threshold, the switch S4 is turned off to protect both the load and the converter from being damaged by such failure. It should be noted that the switch S4 needs to be turned off slowly to avoid causing large voltage spikes, since there is no current path available during protection. When the power supply Vin is desired to be purely disconnected from the LED driving circuit, both the boost converter power switch S1 and the additional switch S4 are turned off slowly in order to avoid large voltage spikes.
Referring now to FIG. 1(b), another prior art solution to implementing high dimming ratio control, short circuit protection, and input disconnection realization is illustrated. Two additional switches S5 and S6 are inserted to a conventional boost converter topology. One end of the switch S5 is coupled to the anode of the series LEDs, and an opposite end of the switch S5 is coupled to ground. The switch S6 is connected between a power supply Vin and an input terminal of the conventional boost converter. In normal operation, the boost converter power switch S1 and rectifier switch S2 are turned on complimentarily, both the additional switches S5 and S6 are in continuous conduction. When dimming, the switch S5 is controlled to be turned on/off by a dimming signal, and the working principle is the same as that of the switch S3 in FIG. 1(a) to realize high dimming ratio control. For short circuit and over current protection, the switch S6 is turned off to cut the power supply Vin from delivering power to the LEDs load. Further complete disconnection of the power supply Vin from the boost converter circuit is achieved to prevent power leakage. However it should be noted that the switch S6 needs to be turned off slowly to avoid large voltage spikes. Otherwise, it is necessary to add a freewheeling diode D2 to create a current path, with an anode of the diode D2 coupled to a common node of the switch S6 and the inductor L1 and a cathode of the diode D2 coupled to ground.
For both the aforementioned solutions of FIG. 1(a) and FIG. 1(b), the additional switches S3 and S4 or S5 and S6 are always on during normal operation, thus extra conduction loss is introduced. Further, when dimming, the additional switches S3 and S5 are on when the dimming signal is on, resulting in extra conduction loss as well. Moreover, when realizing short circuit protection and input disconnection functions, the switches S4 and S6 should be turned off slowly, which needs corresponding control circuitry and thus increases the system complexity. Otherwise, for the solution shown in FIG. 1(b), an additional freewheeling diode is needed, which introduces more cost. In brief, the additional switches used for achieving high dimming ratio control, short circuit protection and input disconnection increase the conduction loss of the system and the additional switches introduce more cost and system complexity.